Optical communications systems using optical fibers have developed rapidly and are extensively used at the present time. Together with the development of optical communications systems, the field of integrated optics has developed. Integrated optics creates opto-electronic components, such as light sources, photodetectors, switches, modulators, amplifiers, and filters, on a common substrate in a manner analogous to silicon integrated circuits. Of course, not all such devices need be present in one circuit.
As might be expected, many opto-electronic components have been developed for both integrated optics as well as for other purposes. For example, optical filters are useful as multiplexers in wavelength division multiplexing systems. Lasers are used as light sources for communications purposes. Modulators have been developed using waveguides fabricated in lithium niobate substrates. Several opto-electronic components will be briefly described.
Ramer teaches a structure that he states to be useful as a switch, modulator, or filter in U.S. Pat. No. 4,679,893 which issued Jul. 14, 1987. The structure had two waveguides that were optically coupled to each other at two positions by optical couplers. Electrodes on the waveguides and the couplers controlled the operation of the structure. Again, the structure could be used as an interferometer, modulator, broadband switch, or a narrow band filter.
Moslehi et al(Moslehi) describe a fiber optic lattice filter in U.S. Pat. No. 4,768,850 which issued Sep. 6, 1988. "Filter," as used in this patent, means filtering of modulation impressed upon a signal as Moslehi is interested in signal processing operations. "Filter" does not mean a structure that is used to filter radiation with respect to radiation. The filters disclosed are limited to fiber optic filters, and there is no discussion of electro-optic tuning of the filter described or of other types of filters.
Kawachi et al(Kawachi) describe a component useful in optical switches in U.S. Pat. No. 5,044,715 which issued Sep. 3, 1991. Special attention is drawn to FIG. 17 which depicts two waveguides that are optically coupled to each other at a plurality of points. The structure depicted is designed and used for broadband purposes; that is, the switching function performed should be wavelength independent. In other words, the desired switching function requires achromatic performance. Other figures of interest include 28 and 33; again, broadband performance is obtained.
Cimini et al (Cimini) describe a wavelength tunable optical filter useful in wavelength division multiplexing systems in U.S. Pat. No. 5,022,730 which issued on Jun. 11, 1991. The filter specifically described uses a distributed Bragg reflector and a high reflectivity mirror. The approach taken by Cimini with a distributed Bragg reflector and a mirror thus differs considerably from the approaches taken in the previously described structures which use waveguides.
The prior art also shows waveguide filters that can be tuned using the thermo-optic effect to differentially change paths of adjacent and optically coupled waveguides thereby producing the desired filtering effect. This technique of thermo-optic heating has the drawback of being both relatively slow and requiring relatively high continuous power dissipation to produce and maintain the required temperature changes. It is noted that there are many applications in which the slow response time is not a significant disadvantage. The prior art also shows, as previously mentioned, waveguides fabricated in electro-optic materials such as lithium niobate. However, multi-stage filters have not been fabricated in such materials. There are several possible explanations as to why such filters have not been fabricated. First, it may have been believed that the required path length difference could not be expeditiously obtained given the constraint of a finite size lithium niobate substrate. Second, it may have been believed that processing variations in fabricating the waveguides were too large for the structures to be useful in these applications.